If someone asks "Can you just..." and you can't do it for some reason, then can you just explain it to them why you can't do it? Getting mad about doesn't solve anything and passes up a great opportunity to fill in their missing information.
An understanding gap is more like a model gap; the people between which the gaps exist usually have entirely different domains with different useful models. An understanding gap might exist between a salesperson and a developer, or a chemist and a biologist; there might be shared reference points to serve as foundations for bridging the gap, but they're much further up the ontological hierarchy, as it were, and may require entire reconstruction from first principles in the worst cases. This leads to interesting conversation and personal development, for sure, but it's also not a way to quickly resolve what may be a pressing issue. Closing an understanding gap might take a four-year degree, or at the very least, weeks or months of reading and learning.
I agree that getting mad about it is not the way to go; but this latter case is where 'just' is most dangerous; to get a bit Rumsfeldian, the unknown unknowns on the part of the 'juster' often run very deep, and would require extensive education to mend.
A bit like asking a golfer why he doesn't just hit the ball straight on the green every drive. It's instinct to attempt, but difficult to explain why it's not going to work. Not to mention if I had to explain every time why I can't just hit the green, I'd get pretty pissed off with people asking!
Therefore, when I'm in a position where I want to throw out a quick "can you just", I try to phrase it differently, because I know how frustrating that comment can be.
I don't see competitiveness here. I want to see my teammates as people and empathize with them as such.
The number of times I have had, over the course of months, to explain again and again why you can't "just" remove feature X from project Y to make it doable in less than D days it is currently estimated to. One day, I would commit to do it because at that moment I didn't remember that specific bit while being quizzed in an unrelated meeting that veered into offtopic armchair solution designing by bored "solution architects" (i.e. middle manager who last looked at code a decade ago)